1. Field of the Invention
The present invention relates generally to read heads for computer data storage devices. In particular, the invention concerns a structure for protecting a read head from electrostatic discharge.
2. Description of the Related Art
Data storage devices, such as magnetic disk drives and tape drives, used to store information for computer systems are well known. In magnetic data storage devices, a medium, such as a magnetic disk platter or magnetic tape, is treated with magnetic material. The magnetic material can be polarized in order to cause phase reversals in a magnetic field to encode information on the medium. The phase reversals used to encode information can be detected by magnetic sensors, commonly referred to as read heads. It is common for a read head to be mounted in a structure, commonly referred to as a slider. Sliders typically fly over the surface of the medium supported by a thin layer of air, commonly referred to as an air bearing. The air bearing is generated by relative motion of the slider with respect to the medium. For example, in a disk drive device, the disk platter is rotated to generate relative motion between the medium and the slider. The slider may be positioned radially over the medium to allow the read head access to any region of the medium as the medium rotates. FIG. 1 is an illustration of a slider 1 having two rails 3. Each rail 3 has an air bearing surface 5. A read head 7 is located on a xe2x80x9cdeposition endxe2x80x9d 4 of each rail. The slider 1 moves in the direction of arrow 9 relative to a magnetic medium.
One well known type of read head is referred to as a magneto-resistive (xe2x80x9cMRxe2x80x9d) head. An MR head uses magneto-resistive material (commonly referred to as an xe2x80x9cMR sensor elementxe2x80x9d) to sense changes in a local magnetic field. FIG. 2 is a simplified illustration of a cross-section of an MR read head 7 within a rail 3 of a slider 1, viewed from the air bearing surface 5. The arrow 9 indicates the direction of the read head 7 with respect to the medium over which the read head 7 flies. An MR sensor element 11 is shown disposed between a first magnetic shield 13 and a second magnetic shield 15. The first and second shields 13, 15 are typically formed of a magnetic material, such as a nickel/iron alloy, which prevents the magnetic fields of adjacent regions of the medium from distorting the fields associated with the information that is being read from the medium. Surrounding each shield 13, 15 and the MR sensor element 11 is an insulating material 17, such as alumina. The insulator 17 prevents the MR sensor element 11 from coming into direct electrical contact with either the first or second shield 13, 15. Also shown in FIG. 2 is a substrate 19. The substrate 19 may be a ceramic material, such as titanium carbide.
FIG. 3 is a cross-sectional view through line 3xe2x80x943 of the read head 7 shown in FIG. 2. The MR sensor element 11 (shown by broken line to indicate that the sensor 11 is obscured by a sensor lead 21) is coupled to additional circuitry, which is well known in the art, by sensor leads 21 (only one such lead 21 is shown on the near side of the MR sensor element 11). A second lead (not shown) is coupled to another side of the MR sensor element 11. A carbon overcoat 20 may be applied to the air bearing surface 5 to minimize wear and protect the relatively soft shields 13, 15 and MR sensor element 11 from damage. The overcoat 20 has little effect on the likelihood that a sparkover will occur at the air bearing surface 5.
One problem with MR heads, such as the head 7 shown in FIGS. 1-3 is that electrostatic charges may be transferred from an external source (such as a human body) to the components of the MR read head 7 (such as the shields 13, 15, MR sensor element 11, and substrate 19) during production. When the charge transferred to one component is sufficiently large, an electrical discharge, commonly referred to as a xe2x80x9csparkoverxe2x80x9d occurs. Such sparkovers are most likely to occur during production and handling of the head 7.
Sparkovers can damage the head. For example, the high current density at the sparkover location typically results in material near the sparkover melting. This damage may occur at the air bearing surface 5 of the slider 1. In a high percentage of MR read heads in which sparkover damage at the air bearing surface 5 occurs, the result of the sparkover damage is either increased resistance, or alternatively, a near open circuit condition in the MR sensor element circuit. In addition, damage to the air bearing surface 5 results in undesirable changes in the flying height characteristics of the slider 1. That is, even the minor changes in the surface characteristics of the air bearing surface 5 have a great impact on the flying height characteristics of the slider 1. Because of the undesirable effects of sparkovers, the manufacturing yield for MR read heads is reduced in proportion to the frequency with which such sparkovers typically occur.
Studies of such electro-static discharges have revealed that these discharges typically occur in one of three regions. These three regions are indicated in FIG. 3 by the letters xe2x80x9cAxe2x80x9d, xe2x80x9cBxe2x80x9d, and xe2x80x9cCxe2x80x9d. As shown in FIG. 3, the regions of discharge are typically along the air bearing surface 5 (even when a carbon overcoat 20 is provided) due to a higher electric field generated in the air bearing.
FIG. 4 illustrates an electrical model of the circuit formed by the elements of the MR head 7. The resistance of the leads 21 to and from the MR sensor element 11 is modeled as two resistors 23, 24. The resistance of the MR sensor element 11 is modeled as a resistor 25. One method for preventing damage due to electrostatic discharge is taught by U.S. Pat. No. 5,272,582, entitled xe2x80x9cMagneto-Resistance Effect Magnetic Head with Static Electricity Protectionxe2x80x9d, issued to Shibata, et. al on Dec. 21, 1993. In Shibata, two sensor element magnetic cores are deposited to form a magnetic gap near the air bearing surface of a slider. The two magnetic cores are in magnetic contact with one another at a xe2x80x9cback gapxe2x80x9d which is away from the magnetic gap. An insulating layer is placed between each sensor element magnetic core at the magnetic gap. An MR sensor element is place between the insulating layers such that the MR sensor element is within the magnetic gap. A ground conductive layer is electrically connected to a first of the magnetic cores to route to ground the electric charges coming into the magnetic gap from the magnetic recording medium. Accordingly, Shibata attempts to keep the magnetic cores which form the magnetic gap at a controlled potential. This arrangement is intended to prevent electric charges that may come from the magnetic recording medium from rushing into the magnetic gap.
A second method for preventing electro-static discharge and the associated damage that such discharge causes is taught in IBM Technical Disclosure Bulletin, Vol. 21, No. 11, dated April, 1979, by Rohen (hereinafter referred to as xe2x80x9cRohenxe2x80x9d). FIG. 5 illustrates the approach taken by Rohen. In FIG. 5, an MR element 31 is located at one end of the structure. A first conductive region 33 and a second conductive region 35 are electrically coupled to a ground potential via terminals 37, 39. An insulating material 41 isolates these regions 31, 33 from two additional conductive regions 43, 45. Regions 43, 45 provide a conductive path for current to the MR element 31. During fabrication, the upper portion 47 of the structure is removed to the broken line 49. By coupling the regions 33, 35 to a ground potential, a low potential point is provided for any direct electrostatic discharges, and the grounded side bars formed by the regions 33, 35 provide a Faraday shield to lessen the effect of indirect electro-static discharges.
A third method for preventing electro-static discharge and the associated damage that such discharge causes, requires providing an alternative path for a sparkover. This method has been used with conventional inductive read/write heads. For example, in a typical inductive read/write head, the inductive coil is greater than approximately 3 xcexcm from the yoke. The dielectric between the inductive coil and the yoke is typically an insulator, such as alumina (Al2O3). A spark gap device is formed which causes a sparkover from the inductive coil or the yoke in order to reduce any electro-static charge that builds on these components. Such a spark gap device is placed close to a component to be discharged. The charge built up on the component will cause a sparkover to the spark gap device at a lower voltage than is required to cause a sparkover to any other component. For example, in a conventional inductive read/write head, a spark gap device would be located approximately 1 xcexcm from the component to be discharged. Thus, a sparkover will occur at a substantially lower voltage than is required for a sparkover across the 3 xcexcm gap between the yoke and inductive coil.
However, because of the relatively short distance between the components of an MR read head, the voltage at which a sparkover occurs between those components is relatively low. For example, the voltage required to cause a sparkover (i.e., the xe2x80x9csparkover voltagexe2x80x9d) between an MR sensor lead and a grounded magnetic shield separated by 0.12 xcexcm is only 60 volts.
In contrast, in a typical MR head (such as the head 7 shown in FIG. 1) the distance between one of the magnetic shields 13, 15 and the MR sensor element 11 is approximately 0.12 xcexcm. Therefore, a sparkover will occur between the magnetic shields 13, 15 and MR sensor element 11 of a conventional MR read head at a far lower voltage than between the yoke and inductive coil in a conventional inductive read/write head. Furthermore, because the sparkover between components of the MR read head 7 can occur though air at the air bearing surface, the required sparkover voltage between the magnetic shields 13, 15 and the MR sensor element 11 is even lower than would be the case if the sparkover had to traverse an insulator. Accordingly, it would be very difficult to develop a spark gap device which would provide an alternative path for discharge of any charge that builds on the components of an MR read head (i.e., a path through which a sparkover can be induced by a weaker electric field than is required to cause a sparkover between the components of the MR read head). For example, voltages in excess of 1000 volts are required to cause a sparkover between a yoke and inductive coil in a conventional inductive device. In contrast, 60 volts can cause a sparkover between an MR sensor lead and a grounded magnetic shield separated by 0.12 xcexcm. This difference is due to the relatively short distance across the gap between the MR sensor lead and the magnetic shield, and also due to the fact that the components of an MR read head are essentially exposed to air at the air bearing surface. The dielectric constant for air is such that sparkover will occur at lower voltages across air than across many other materials.
While the solutions provided by Rohen and Shibata reduce the chance of damage to an MR read head occurring, damage due to sparkovers (particularly at the air bearing surface) remain a persistent problem which undesirably effects manufacturing yield. Accordingly, it is an object of the present invention to provide a structure that is less susceptible to harmful sparkovers at the air bearing surface. Another object of the present invention is to provide an inexpensive structure which is less susceptible to damage from sparkovers at the air bearing surface. Still another object of the present invention is to provide a method for efficiently fabricating a structure that is less susceptible to damage from sparkovers at the air bearing surface.
The present invention is a magneto-resistive read head used to sense magnetic fields which emanate from a magnetic storage medium, such as a platter of a computer disk drive device or magnetic tape used in a tape drive. In accordance with one embodiment of the present invention, xe2x80x9cparasitic shieldsxe2x80x9d are placed in close proximity to magnetic shields of a read head. The gap between a parasitic shield and a magnetic shield is preferably narrower than the gap between a magnetic shield and either the substrate on which the read head is formed, or a sensor element. Accordingly, a parasitic shield provides an alternative path for currents associated with sparkovers, thus preventing such currents from damaging the read head.
Each of the parasitic shields is electrically coupled to the sensor element through a resistive element. Therefore, the electrical potential of parasitic shield will be essentially equal to the electrical potential of the sensor element. Accordingly, if charges accumulate on the magnetic shield, current will flow to the parasitic shield at a lower potential than would be required for current to flow between the magnetic shield and the sensor element. Alternatively, the parasitic shields may be directly electrically coupled to a structure of known electrical potential, such as the substrate.
In accordance with a second embodiment of the present invention, conductive spark gap devices are electrically coupled to sensor element leads and to each magnetic shield. Each spark gap device is brought into very close proximity of the substrate to provide an alternative path for charges that build up between the sensor element and the substrate to be discharged. In accordance with one embodiment of the present invention, the spark gap devices are fabricated at the wafer level on a deposition end of a wafer of semiconductor substrate material using photolithography and masking techniques. In one embodiment of the present invention, pads at the deposition end may be connected to the substrate and shields to allow external connections to be made.
In the preferred embodiment of the present invention, the ends of the spark gaps that are brought into close proximity with the substrate are configured with high electric field a density inducing structures which reduce the voltage required to cause a sparkover between the spark gap device and the substrate. Alternatively, the spark gap devices may be directly coupled to the substrate and brought into close proximity with the magnetic shields and the sensor element.
The details of the present invention, both as to its structure and operation, can best be understood in reference to the accompanying drawings, in which like reference numerals refer to like parts, and in which: